My expiration date on my apple pay credit card is not updating on my iphone

Mine hav always updated automatically. See the following from Change or remove the payment cards that you use with Apple Pay - Apple Support

Update your Apple Pay card number and expiration date

With Apple Pay, your payment card has a unique Device Account Number used to process payments and an expiration date associated with that Device Account Number.

If your card expires or you otherwise get a new card:

• The card number and expiration date linked to the Device Account Number should be updated automatically. 
• If your card issuer doesn’t support these updates, you might need to remove the card and add it again.